It is often necessary to produce Schottky barrier diodes of closely controlled breakdown voltage. The breakdown voltage is controlled by the thickness of an epitaxial layer on a gallium arsenide substrate, and the breakdown voltage is reduced from a high level to the desired level by reducing the thickness of the epitaxial layer. Such thinning can be accomplished by immersing the wafer containing the epitaxial layer, in an electrolyte of nearly neutral pH and passing a current from the substrate through the layer and into the electrolyte. The current oxidizes atoms at the face of the layer that is closest to the electrolyte, and this oxide can be washed away to produce a thinner layer. However, it is difficult to determine when the thinning should be stopped to achieve the desired breakdown voltage in diodes constructed from the wafer.
The oxide film formed during anodic oxidation is electrically insulative. Accordingly, when the voltage applied between the wafer substrate and electrolyte reaches a predetermined maximum value, the current flow progressively decreases as the oxide thickness grows, and therefore the rate of thinning decreases. To increase the thinning rate, the wafer can be removed and cleaned of oxide, and then again immersed in electrolyte and thinned again. However, such repeated removals complicate and lengthen the thinning process. Epitaxial thinning processes which facilitate the monitoring of the breakdown voltage characteristics of the epitaxial layers, and/or which simplify and speed up the layer thinning, would be of considerable value.